Isolating Transformer (UK)

[cprobertson1]

Hey folks!

   I'm in need of an isolating transformer for doing electrical repairs on live equipment - but I can't seem to find one (for the UK anyway!)

   Any ideas on where I can buy such a unit?

    Is it worth just building one? (and in that case, do I actually need to fork out for a full 3000kVA transformer (£165!) that would cover the max 13A from a plug socket, or do you reckon I'd get away with a lower power rating, perhaps the in 500-1000VA region?

   Many thanks folks!

[madires]

I got some questions too Do you need just an isolation transformer or an isolation transformer with a variac? What is the maximum power requirement of the devices you want to repair?

[cprobertson1]

Just regular isolation! (no variac).

Generally speaking it's small household items - off the top of my head, the most power-hungry thing I've repaired was a 400W television - I don't generally get things with motors or heating elements in them so the vast majority of stuff I repair is far below that!

I can get one built up from http://www.airlinktransformers.com/ with a 750VA toroidal transformer, MCB on the output with surge protection in an IP65 casing for £168ish (which is only a little more than the cost of a 3kVA transformer on it's own!)

I can shave £20 from it by dropping the output circuit breaker (I'll generally use a plug-in RCD anyway so it may not even be necessary)

[Richard Head]

If you are making a custom setup then also consider adding inrush limiting for the Variac. Variacs are notorious for tripping breakers on start-up due to their high remanent  flux caused by their tapewound structure. The output breaker is a good idea.

[cprobertson1]

If you are making a custom setup then also consider adding inrush limiting for the Variac. Variacs are notorious for tripping breakers on start-up due to their high remanent  flux caused by their tapewound structure. The output breaker is a good idea.

Oops! I didn't word my previous spec-list very well - inrush current / surge protection included on the input! - I think I'll keep the circuit breaker (as you said, it's a good idea!) - for the sake of £20 it's not much!

-750VA transformer (230V to 230V)
-UK plug w/ >=2m cable on input
-input single pole circuit breaker
-input surge protection
-UK socket on output
-2 pole circuit breaker on output
-IP60 or greater (dust proof) GRP case with handle

[madires]

Yes, a current inrush limiter for a toroidal transformer above 500VA is strongly recommended. Also spend the few extra bucks for the output circuit breaker. The RCD would only work for the primary winding, since the secondary is isolated 170 quid for the complete box seems fine to me.

[cprobertson1]

The RCD would only work for the primary winding, since the secondary is isolated

  - let's pretend I never said that

£170, delivered doesn't seem too bad tbh, I just like being stingy!

[nctnico]

I recommend against using an isolation transformer and get a good GFI/RCD instead. The chance you accidentally ground a piece of equipment by connecting test equipment (for example an oscilloscope) or something else to it is huge and this situation will make things worse where it comes to safety. Better get a differential probe with the right CAT ratings if you need to measure signals using an oscilloscope.

There have been several threads about isolation transformers and their inherent dangers so please read those. An isolation transformer is a stop-gap solution from the past!

[RGB255_0_0]

You have many more ways to kill the power from the mains on an RCD-protected circuit if you were to get into contact with a live wire.

Isolation transformers make some people think they are safe and forgo some precautions - such as using only one hand. If you were to get into circuit with an isolation transformer and pass enough current that exceeds your let-go threshold, there is almost no hope you can break the circuit on your own as there is no leakage.

At least with mains you could get pushed back onto a heating pipe or touch something else grounded to break the power.

Proper probes is the way forward.

[cprobertson1]

If you get one built you may as well get a 120V tap on it so you can quickly use it as a stepdown transformer. You also need to consider what ground/earth wiring you have present at the output.

That's a good shout! Having said that, I'm not sure how often I'd actually use the 110/120V output - but hey! The cost is negligible, so I might as well - never know when I might need it!

Would I be right in saying that the earth should be carried straight through from the input to the output side, with no direct connection to the neutral on the output (obviously with the TNC-S wiring in the house it will be bonded to neutral at the distirbution box of course)

You have many more ways to kill the power from the mains on an RCD-protected circuit if you were to get into contact with a live wire.

- But with an isolated transformer, you'd need to go out of your way to touch both live an neutral on the secondary side to get a shock - at which point the MCB should kick in (though you'll still get a jolt).

My understanding is that: yes, in principle there are more ways to cut the power in the event of a shock without the isolation; but the likelihood of a shock decreases dramatically with isolation in place; as long as you follow correct safety precautions (which you should be doing anyway!).

Obviously, if you're going in with both hands and touching things all willy-nilly then the risk of a lethal shock shoots up again - though if you're the sort of person to forego safety precautions then you probably shouldn't be playing with mains in the first place

Proper differential probes would of course be awesome, but I can't afford them - they'd actually cost more than my scope!

And even with an RCD in place, if you brush against a live wire or chassis while probing you'll make a path to earth via your body and you'll get a shock for at least 40 to 100 milliseconds before the RCD trips - which certainly gives you a fighting chance; but still, it'd be better to prevent a shock entirely

With isolation, the live is isolated from earth and its difficult to complete that pathway unless there is already an earth loop fault in the system (which you should have tested for before plugging it in! (i.e, as mentioned before, you need to be in contact with both live and neutral on the secondary - in this case we're using the earth loop to help us electrocute ourselves))

Feel free to correct me if I'm wrong though!

[RGB255_0_0]

Absolutely. As long as you know what you're doing you will always be fine, mains-isolated or not. Obviously if you've just drank 3 Red Bulls or 3 shots of Espresso you probably shouldn't be anywhere near live circuitry.

But "but the likelihood of a shock decreases dramatically with isolation [transformer] in place" I don't agree with. Wearing proper gear dramatically decreases shock hazard, not the isolation transformer. Test the RCD before working on the DUT. If you work on live equipment regularly, replace them regularly. Isolating yourself does the same job but if you happened to be in direct circuit with neutral and live, at least the circuit with RCD protection is there.

[RGB255_0_0]

http://www.hse.gov.uk/pubns/indg354.pdf

Supplies to equipment under test
Provide each item of equipment under test with its own test supply. These supplies
should be from designated sockets or terminals provided with covers, interlocked
with the supply isolator. The supplies should have suitable system protection
against overload and overcurrent in the event of faults, eg fuses. Note that:
? where an isolating transformer is used for the supply to the equipment under
test, it should comply with BS 61558 and a separate transformer used at every
test bench. If this is not reasonably practicable, the same isolating transformer
may be used for supplies to alternate benches, provided the risk of referencing
this supply to earth at any bench is properly controlled and the transformer
does not then have an unacceptably high leakage current;
? the supply from the isolating transformer should be provided from a single
socket outlet and clearly marked ‘only for use for making live equipment under
test’. No fixed wiring should be connected to the earth terminal of the outlet
socket. The face plate of the socket should be made of insulating material.
There must be no unnecessarily exposed live parts on equipment under test;
? in certain circumstances Class I equipment under test must be effectively earthed
unless supplied via an isolating transformer. This will bring with it an increased risk
of electric shock which may be minimised by using other precautions.
? when the equipment under test is Class I, any pre-existing earth fault must be
detected and corrected before making the equipment live. In the case of the
supply from an isolating transformer, failure to do this will mean that there may
be a hazardous shock risk, if there is a simultaneous contact between the
enclosure of the equipment and one or both poles of the isolated test supply;
? the integrity of the circuit protective conductor (earth) of all portable/
transportable Class I equipment must be re-tested after all test-bench work has
been completed, to make sure there are no earth faults before the equipment is
used again on a normal mains supply.

[MrSlack]

I don't use an isolation transformer when working on live equipment. I have a strict policy of doing the following though which is adhered to religiously:

1. Unplug, isolate.
2. Attach test probes/equipment.
3. Plug in, power up.
4. Observe measurement without touching the unit.
5. Goto 2.

If your measurement equipment is properly earthed and PAT tested it should be fine.

I don't own differential probes nor do I want to spend any cash on them, so I used an inexpensive AD8130 with two normal 10x scope probes and a secondary switchable attenuator. Works fine, CMRR drops off pretty quick at about 50MHz and noise creeps in so you need to use LP filter in trigger and it'd probably explode if you put more than 500v diff through it but it does the job well enough. I'll detail the design at some point. Cost about £10 in total and runs off a 9v battery. The worst possible current leakage from mains is about 500uA with the design.

[madires]

... or use the poor man's differential scope probe, i.e. two channels, ground leads removed, ch1 plus ch2 inverted and stay within the voltage limits of the scope.

[MrSlack]

That works fine for low frequency stuff, under about 20MHz. 3dB point on my 465B with some basic no brand probes is about 15MHz. Above that you need short probes; bring the amp to the signal rather than bringing the signal to the amp. 3dB point is about 70MHz now using some chopped down no brand probes and my diff probe amp.

[Ian.M]

Most of the personal safety benefits of an isolating transformer are moot assuming your bench supply is effectively RCD protected.  Also there is the issue that an isolating transformer doesn't detect a single fault to ground so any mistake, or even normal usage can lead to a false confidence that it is protecting the user.  A MCB on the output provides no protection against shock as the trip current is far far higher than that likely to cause electrocution, so its only useful to prevent equipment damage and fire.

However an isolating transformer is helpful when one is working on the primary side of SMPSUs,  non isolated PSUs etc. especially when working with more complex devices that require several steps to set up the desired operating conditions starting from fully powered down.   If you pre-attach suitable test points to clip the probe to so you do NOT have to hold the probe while making a measurement, you can even probe around the circuit with power on without excessive risk.

Differential probing is another matter - its rarely needed for working on PSUs, and usually Ch1-Ch2 mode is good enough.

[cprobertson1]

Most of the personal safety benefits of an isolating transformer are moot assuming your bench supply is effectively RCD protected.

See, my problem is that I'm repairing household goods: and getting all up inside them Depending on the product I can sometimes get away with just powering it from a bench supply and bypassing the mains side of things entirely - other times its not convenient to do so, or there are too many rails for me to power manually (rarely), or the problem is with the PSU itself - and of course, sometimes you just need more than the 60V my bench PSU can supply!

Its for cases like that I was thinking the isolated transformer would be invaluable - is that the case? Or would a good RCD a better option?

at which point the MCB should kick in (though you'll still get a jolt).

A MCB on the output provides no protection against shock as the trip current is far far higher than that likely to cause electrocution, so its only useful to prevent equipment damage and fire.

Aye! I realised that a little after I said it - a typical MCB trips at what, 3 Amps? 5? 13? Even with a 1-amp MCB, if you get to the point where you have an ampere flowing through you - cutting the power isn't going to help much (unless you happen to have a fondness for BBQ Engineers).

It doesn't take much to trigger fibrillation either - something meager like 30mA if I recall! (which, come to think of it, is below the tripping current for most RCDs - which, surprise surprise, is why you take precautions to avoid getting a shock in the first place )

[Ian.M]

I'm *NOT* talking about bench DC PSUs.  The AC mains supply to your bench should be protected by a 10mA RCBO.   Its a PITA if bench PCs, test & measurement equipment etc. and lighting are on the same RCBO as the D.U.T. and it also restricts the maximum safe current available to the D.U.T., so I'd suggest two 10mA RCD protected 13A 2.5mm^2 feeds to the bench, one for D.U.T. (with or without  the isolating transformer) and the other for all other bench equipment.

The only other practical alternative for more than 13A total bench load is to wire your bench as a 32A radial circuit from the main consumer unit, (possibly via a 32A socket so the licensed electrician doesn't have to sign off on your actual bench's wiring), to a sub-consumer unit at the bench, which lets you use 10mA DIN rail RCBOs for the individual bench circuits, + have a master switch to isolate the bench.

[Delta]

Most of the personal safety benefits of an isolating transformer are moot assuming your bench supply is effectively RCD protected.

See, my problem is that I'm repairing household goods: and getting all up inside them Depending on the product I can sometimes get away with just powering it from a bench supply and bypassing the mains side of things entirely - other times its not convenient to do so, or there are too many rails for me to power manually (rarely), or the problem is with the PSU itself - and of course, sometimes you just need more than the 60V my bench PSU can supply!

Its for cases like that I was thinking the isolated transformer would be invaluable - is that the case? Or would a good RCD a better option?

at which point the MCB should kick in (though you'll still get a jolt).

A MCB on the output provides no protection against shock as the trip current is far far higher than that likely to cause electrocution, so its only useful to prevent equipment damage and fire.

Aye! I realised that a little after I said it - a typical MCB trips at what, 3 Amps? 5? 13? Even with a 1-amp MCB, if you get to the point where you have an ampere flowing through you - cutting the power isn't going to help much (unless you happen to have a fondness for BBQ Engineers).

It doesn't take much to trigger fibrillation either - something meager like 30mA if I recall! (which, come to think of it, is below the tripping current for most RCDs - which, surprise surprise, is why you take precautions to avoid getting a shock in the first place )

Typical MCBs are 6A, 10A, 16A, 20A etc.

All RCDs used to protect against direct contact MUST trip by 30mA (for the reason you alluded to, even though you didn't realise).

Mate, no offence, but given your lack of understanding of mains protective devices etc, do you think you should be prodding around in live equipment?

[cprobertson1]

No offence taken

You're very right in your concern though! I've been thinking that a bit to myself as I've been replying to things on this thread if I'm being honest.

On the other hand - I generally power things from the bench supply if I can - it's a rare occurrence that I actually have something live exposed on the bench - the problem being that sometimes something dicky comes through that needs to be probed while either while powered up, or even while powering up - which was why I was inquiring about extra protection

--EDIT--
In fact, given the rarity of having to actually deal with live mains - I could probably get away with just refusing to repair anything I can't power safely from the bench PSU. That's probably a safer option overall.

[RGB255_0_0]

You're talking of uA to send your heart into fibrillation. Let-go threshold is way higher and depends on body mass though.

[Delta]

No offence taken

You're very right in your concern though! I've been thinking that a bit to myself as I've been replying to things on this thread if I'm being honest.

On the other hand - I generally power things from the bench supply if I can - it's a rare occurrence that I actually have something live exposed on the bench - the problem being that sometimes something dicky comes through that needs to be probed while either while powered up, or even while powering up - which was why I was inquiring about extra protection

--EDIT--
In fact, given the rarity of having to actually deal with live mains - I could probably get away with just refusing to repair anything I can't power safely from the bench PSU. That's probably a safer option overall.

The best piece of practical advice in this thread was to hook up your test probes with the DUT powered doon, then switch it on, take measurements, switch off, move probes etc...

I am no self-righteous "I ken better than you" type; I possibly owe my life to RCDs...     I spent years working on 24VDC instrumentation, and would probe around live panels will nilly, I then found myself being equally blasé when I had to go into a 110VAC relay logic panel, and a 415 or 690V TP starter panel.  I've had electricians grab "that daft ET" away from a panel and have a word. 
You just can't "get away with it" with mains.  This shit bites.

If you have to work on live mains equipment (and some times you do), do as mentioned above, and throwing a 5mA trip RCD (used on medical gear) on the supply wouldn't be a bad addition.

Stay safe, Brother.   

[cprobertson1]

The best piece of practical advice in this thread was to hook up your test probes with the DUT powered doon, then switch it on, take measurements, switch off, move probes etc...

Aye, that's basically what I've been doing - I'm not the kind to go taken chances if I can help it

This shit bites.

No kidding! Though I've met a surprisingly large number of electricians who say they've had a shock before.......

If you have to work on live mains equipment (and some times you do), do as mentioned above, and throwing a 5mA trip RCD (used on medical gear) on the supply wouldn't be a bad addition.

Stay safe, Brother.   

Any idea where the best place to purchase a low trip-current RCD like that? All the plug-in types I can find are 30mA, and I can only find RCDs as low as 10mA trip current (for DIN rail mounting)(but that's just checking Farnell and RS-components mind you)

PS - just out of curiosity, why are RCBOs so expensive?

[Ian.M]

5mA RCDs are mostly a US thing and are very rare in the UK, due to the UK's long history of an adequate PE on all socket outlets.  They are reported to be extremely prone to nuisance tripping.   10mA 230V RCDs are available - albeit a little difficult to find compared to 30mA RCDs.  e.g. https://www.amazon.co.uk/white-portable-adaptor-active-non-latching/dp/B00SVED44G should be suitable for field use
However, for a fixed bench installation, I would much prefer a reputable brand of DIN rail RCBO (or RCD if there is already MCB overload protection) + a suitable enclosure for them.

[SteveyG]

Most of the personal safety benefits of an isolating transformer are moot assuming your bench supply is effectively RCD protected.

RCDs are not suitable as primary protection against electric shock. Sure they help reduce the risk of death, but people have still gone into fibrillation from contact with an RCD protected circuit. A 'working' RCD only guarantees a maximum disconnection time not any kind of current limit.

Isolation transformers however are a very useful tool providing you understand what you are doing. I would never substitute working on some live gear with an RCD instead of an isolation transformer - they just aren't reliable enough.

RCBOs are not that expensive, but I would assume mainly because they are a bit more complex to manufacture and have quite a bit going on inside.

[Ian.M]

Unfortunately, its common practice to connect grounded testgear to a D.U.T. fed from an isolating transformer.   As soon as you introduce an earth downstream of the isolation, you are totally unprotected against shock, even if the isolating transformer is fed from a RCD.  Also although a floating isolated supply is safe under single fault conditions, it is unlikely to have automatic earth fault detection so there is a high risk of continued use until a second fault occurs.  The fact that you can touch one (LF) node of a circuit fed from a fully floating  supply with no immediate consequences promotes carelessness and unsafe working practices.   It is much much harder to intentionally or accidentally defeat the protection offered by a RCD and their use does not encourage unsafe working practices.

[SteveyG]

Unfortunately, its common practice to connect grounded testgear to a D.U.T. fed from an isolating transformer.

That simply comes down to stupidity.

[madires]

Don't rely on a single protection method! It's not isolation transformer vs. RCD, it's the question how we can add several layers of protection:
- a RCD is a good idea anyway (though being defeated by an isolation transformer)
- an isolation transformer (more reliable than a RCD and less painful)
- use isolated T&M (or differential probes for your scope)
- rubber gloves?
- procedures (as described in another post)
- check the protection devices regularly

[cprobertson1]

So should I be forking out for an isolation transformer: or will a dedicated 10mA RCD be enough, bearing in mind that it's fairly rare that I have to actually deal with a mains potential circuit and that I obviously follow general safety procedures* when doing so.

*Which I'll probably revise with a qualified electrician in the near future, just to be certain I'm not doing anything unsafely and not realising it - better safe than sorry!

[MrSlack]

From experience, a qualified electrician in the UK is the last person you want to ask about electrical safety.

[cprobertson1]

From experience, a qualified electrician in the UK is the last person you want to ask about electrical safety.

HA!

Good point 

In fact, to quote myself,

Though I've met a surprisingly large number of electricians who say they've had a shock before.......

'Nuff said

I was thinking along the lines of the HV technicians at my place of work who regularly deal with the 450V/600V CNC machines - but then again... they're probably not the best folks to ask either... I've seen some of them do.....things...

Take it the HSE manuals are the way to go?

[nctnico]

Most of the personal safety benefits of an isolating transformer are moot assuming your bench supply is effectively RCD protected.

RCDs are not suitable as primary protection against electric shock. Sure they help reduce the risk of death, but people have still gone into fibrillation from contact with an RCD protected circuit. A 'working' RCD only guarantees a maximum disconnection time not any kind of current limit.

Isolation transformers however are a very useful tool providing you understand what you are doing. I would never substitute working on some live gear with an RCD instead of an isolation transformer - they just aren't reliable enough.

The problem is in the understand what you are doing part. An RCD/GFI at least prevents a lethal/prolonged current to flow. An isolation transformer can easely be defeated by grounded test equipment or give a false sense of safety. The only safe way to work on mains (or any high voltage circuits for that matter) is to use isolated probes and never ever touch the circuit.

[Ian.M]

Personally, I'd have both isolating transformer and high sensitivity  RCD, and also an insulating floor mat (under an ESD mat if you need that) + a bench that does NOT have a metal frame.  All sockets on the bench apart from the isolating transformer output should be RCD protected.

Most of the time in the TV shop I used to work in, a 250VA isolation transformer was sufficient.  A 500VA one will certainly suffice for everything except heating/cooking appliances and those with large motors.  An isolating transformer, used intelligently, improves your safety when working on the low voltage side of equipment with an open frame mains PSU or one integrated on the same board.  However you must always be aware of anything that introduces a ground - which would trip the RCD without the isolation transformer - as that instantly removes the protection offered by both.

[Mark Hennessy]

In an electronics workshop - which might be slightly different to what the OP is doing - then there is only one use of an isolation transformer: to allow the safe connection of test equipment to a DUT. The classic example has already been given; the primary side of a switched-mode PSU (where the -ve output of the input bridge is a negative-going half-wave waveform wrt mains earth with a peak value of -325V). In days gone by, prior to baseband video/audio inputs, TV sets had a live chassis, so also needed to be isolated for the same reason.

Note that "allowing safe connection of test gear to the DUT" is nothing to do with the probability or magnitude of electric shocks - the transformer is not intended to change that. Naturally, as soon as you've connected test gear, the DUT is no longer floating, so it remains hazardous; but the transformer has allowed you to tie it to ground at the point you need to make a measurement...

A really important rule - not yet mentioned, I think - is this: only one DUT may be connected to the transformer.

Also, the mains earth should not be carried through to the outlet. If you have a DUT with a N-E swap, the case becomes live.

The default working practice it to power a DUT from an RCD, and only use the isolation transformer for those occasions described above. Naturally, when using the isolation transformer, any RCD in line becomes redundant, but doesn't hurt.

Away from all that, if you wish to use an isolation transformer to reduce the magnitude of an electric shock, then you need a centre-tapped secondary, and you ground the centre-tap. This gives you two lines at 115V wrt mains earth. And you should be able to use an RCD at the output (but check it works properly in that mode). But naturally, this supply is no longer floating, so can't be used to safely connect test gear to a DUT as described above.

Personally, I doubt the utility of this arrangement in an electronics workshop environment, and have never used it myself, but it might be appropriate for the electrical repairs that the OP does? Of course this scheme is used on building sites - those yellow-cased "site transformers" produce 110V from a transformer with a grounded centre-tap, so the maximum voltage wrt mains earth is 50V.

I've attached a BBC EGN which might be of interest.

All the best,

Mark

[nctnico]

Another problem I see is distance to the distribution/switch board comes into play with ground protection and that can add delay. Making an additional ground/earth is a no according to some wiring regulations (depends where you live of course), they are also a proximity hazard as well.

Now you are not making sense. The delay is in the order of nanoseconds where the speed of impulses through your nerves is in the several tens of meters per second ballpark so it takes a pulse to go from your brain to your toe tens of milliseconds.

@Mark Hennessy: TVs with a hot chassis are something from the 70's maybe 80's which shows how antiquated using an isolation transformer is. Nowadays just use a differential probe and be done with it. With multiple differential probes you can look at several points in the primary part of a PSU if you like.

[SteveyG]

The problem is in the understand what you are doing part. An RCD/GFI at least prevents a lethal/prolonged current to flow. An isolation transformer can easely be defeated by grounded test equipment or give a false sense of safety. The only safe way to work on mains (or any high voltage circuits for that matter) is to use isolated probes and never ever touch the circuit.

No it doesn't. Conditions can be such that you sustain an electric shock with a current that is not sufficient to trip the RCD, yet be greater than the threshold to cause muscles to contract or be enough to cause cardiac arrest. An RCD won't trip at 20mA, but you'll have a hard time letting go and will lead to death.

[nctnico]

That is easely solved by using a more sensitve RCD and not a reason to disqualify any RCD.

[cprobertson1]

Now you are not making sense. The delay is in the order of nanoseconds where the speed of impulses through your nerves is in the several tens of meters per second ballpark so it takes a pulse to go from your brain to your toe tens of milliseconds.

Sorry, I'm not understanding why you brought up nerve impulses?

Looking at this from a biological perspective, an electric shock usually passes through your lovely salty electrolytic fluids that are between your cells rather than charging through the oily cells (cells have lipid bimembranes which are surprisingly good insulators: so much so that many cells use them to generate a potential difference by separating +ve and -ve ions onto opposite sides of it - similar to charges on a disc capacitor!) - nerves don't provide a particularly good pathway for electricity to flow, and are certainly no more conductive than regular tissues - your circulatory system is the best candidate for passing current from one pace to another in your body (as blood contains lots of salt and water - though I've never actually measured the resistance of a droplet of blood... I might try that next time I cut myself, and perhaps observe it under a microscope while I pass current through it to see what happens....)

Please note that this assertion that neurons are bad conductors in the case of electrocution is speculative (based on my being a qualified biochemist) - but believe it or not, most biochemists only care about the macro-effects of being electrocuted - not what's actually happening at the microscopic level (i.e where the current is actually flowing and how and why)


(Did you know that nerve impulses are actually mostly chemical? An electrical signal (or more specifically, as each cell is discrete, a potential difference across the membrane of the initiating cell) - which triggers voltage-gated calcium channels which then open up - allowing for the uptake of calcium from the synaptic cleft (the gap between two synapse) - the high Calcium concentration within the cell triggers the release of neurotransmitters into the synpatic cleft, which then binds to chemical receptors on the neighbouring neuron, which can then trigger one of several responses - the traditional nervous impulse involves the uptake and release of certain ions within that cell, which triggers that cell to repeat the process with it's neighbour, and so on and so forth (and at the end of the impulse there is generally a chemical feedback loop which amplifies the (chemical) signal into an actual affect - this is obviously an undetailed summary - there's a lot more to it of course! (my degree was in Biochemistry, specialising in Immunology - that was before I became a tooling engineer though!))

[Delta]

No it doesn't. Conditions can be such that you sustain an electric shock with a current that is not sufficient to trip the RCD, yet be greater than the threshold to cause muscles to contract or be enough to cause cardiac arrest. An RCD won't trip at 20mA, but you'll have a hard time letting go and will lead to death.

I assume you mean a 30mA RCD won't trip at 20mA?

[nctnico]

Now you are not making sense. The delay is in the order of nanoseconds where the speed of impulses through your nerves is in the several tens of meters per second ballpark so it takes a pulse to go from your brain to your toe tens of milliseconds.

Sorry, I'm not understanding why you brought up nerve impulses?

Just to outline that the 'systems' in your body that may be affected by a shock are extremely slow compared to the speed at which electricity flows through a conductor. The delay caused by wiring to detect a leakage by an RCD is totally insignificant.

[cprobertson1]

Ah, got you! I misinterpreted it at first - sorry!

The speed of conduction through a human is still pretty high though - the extracellular matrix and circulatory system in particular are moderately good conductors (compared to other tissue that is - they're terrible conductors compared to "normal" conductors!) - but the current speed is still pretty fast.

As you mentioned though - the time delay caused by longer wiring should be fairly small? Might Shock mind clarifying the effect he mentioned (i.e how it works ?)

[Ian.M]

I suspect Shock has confused different types of breakers.
For MCBs, there is an issue with wiring length (actually total loop impedance of the L conductor from the distribution transformer and E back to) when clearing a hard 'bolted' fault to P.E.  Within their max clearing current rating, a lower loop impedance results in a faster trip.

However modern RCDs almost invariably use a balanced L-N current transformer for leakage current detection, and are unaffected by any wiring impedance that doesn't interfere with normal operation of even low wattage loads.

The other issue of relying on a single mechanical device for safety isn't strictly accurate.  At least in the UK, there are now very few ring circuits with 13A outlets that aren't RCD protected at the consumer unit, usually by a whole house 30mA RCD or a 32A/30mA RCBO just for that ring.  So, assuming the bench has local RCD protection, both it and the consumer unit RCD would have to fail, + the user would have to fail to follow safe working practices before there would be a significant risk of shock with serious consequences.

[Delta]

Shock is getting confused and thinking of the operation of Overcurrent Protective Devices (MCBs, fuses...).  The speed at which these will disconnect a hard earth fault and thus protect against indirect contact depends upon the fault loop impedance, and thus the length of the cable run (and conductor CSA) will affect the speed of operation.  all of which is completely irrelevant when talking about RCDs.  If current flows at 0.7 * the speed of light in copper and 0.6 * the speed of light in human tissue; so fecking what! 


EDIT - Beaten to it by less than a minute!

[MrSlack]

To quote my engineering manager about 18 years ago:

"technology (GFI/RCD etc) doesn't solve a whole lot of electrical safety problems. It just stops the next person who comes along who stupidly tries to drag your dead body off the bench from getting toasted too"

[nctnico]

Well, don't take my word for it:
www.tek.com/dl/51W_10640_1.pdf

Oh, and I do take safety very seriously. When I was a teenager a good friend of mine electrocuted himself while tinkering with electricity.

[Delta]

Well, don't take my word for it:
www.tek.com/dl/51W_10640_1.pdf

Oh, and I do take safety very seriously. When I was a teenager a good friend of mine electrocuted himself while tinkering with electricity.

That says not to use an isolation transformer to float the instrument, it absolutely does not say that an isolation transformer should not be used to supply the DUT!

VERY important distinction!

[cprobertson1]

VERY important distinction!

Lol, even I know that one

We really need a sticky post outlining the facts and issues of bench safety in the first post, this comes up all too often and we get people like the "I touch the live mains all the time and I don't die" guys who just distort reality to new players.

That would be a very good idea - especially given how easy it is to get seriously injured (and/or killed) if you mistreat it =/

[nctnico]

Well, don't take my word for it:
www.tek.com/dl/51W_10640_1.pdf

Oh, and I do take safety very seriously. When I was a teenager a good friend of mine electrocuted himself while tinkering with electricity.

That says not to use an isolation transformer to float the instrument, it absolutely does not say that an isolation transformer should not be used to supply the DUT!

VERY important distinction!

You didn't read far enough! In the table on the next page it says not to use an isolation transformer to make floating measurements -period-.

[Mark Hennessy]

Well, don't take my word for it:
www.tek.com/dl/51W_10640_1.pdf

Oh, and I do take safety very seriously. When I was a teenager a good friend of mine electrocuted himself while tinkering with electricity.

That says not to use an isolation transformer to float the instrument, it absolutely does not say that an isolation transformer should not be used to supply the DUT!

VERY important distinction!

You didn't read far enough! In the table on the next page it says not to use an isolation transformer to make floating measurements -period-.

Says the people who sell differential 'scope probes

I think the advice in the attachment I provided is perhaps more balanced, especially as it was written by someone who didn't want their employees to come to harm, rather than a marketing department...

The problem is not the transformer - the problem is education.

[nctnico]

I've read that BBC document but it is basically trying to regulate stupidity (also note it refers to 'rules' from 1989 which is more than 25 years ago). What is more simple than: don't touch it when powered/charged and use a differential CAT rated probe to measure? I really don't see why people still try to defend the use of isolation transformers with more do's & don'ts than the 10 commandments which people also don't follow.

[Mark Hennessy]

I've read that BBC document but it is basically trying to regulate stupidity (also note it refers to 'rules' from 1989 which is more than 25 years ago).

So what has changed in the last 25 years that is relevant to this? We still have Live, Neutral and PE as far as I know.

What is more simple than: don't touch it when powered/charged and use a differential CAT rated probe to measure?

We get that you like differential 'scope probes, but they are relatively expensive, and aren't to be found in every repair and maintenance workshop. And they have problems of their own.

I really don't see why people still try to defend the use of isolation transformers with more do's & don'ts than the 10 commandments which people also don't follow.

1. Only one DUT connected to the output.
2. Don't carry the earth through to the output socket.

Pretty sure that's fewer than 10

As I keep trying to indicate, the only problem with isolation transformers is people's understanding of them. In an electronic workshop, they are about allowing the safe connection of earthed test gear to a DUT - nothing more, nothing less. They are not about changing the probability or severity of an electric shock.

Default live-working practice is to use an RCB (and an MCB for over-current, obviously). The use of an isolation transformer is an occasional requirement for those specific tasks that require it (live-side of a SMPS being the classic example). A differential probe might do a reasonable job as well, but it's an either-or choice - there's nothing wrong with (correctly!) using an isolation transformer.

[nctnico]

I really don't see why people still try to defend the use of isolation transformers with more do's & don'ts than the 10 commandments which people also don't follow.

1. Only one DUT connected to the output.
2. Don't carry the earth through to the output socket.

Pretty sure that's fewer than 10

Then you should apply for a job at the BBC because you can reduce a 9 page document about safety to two sentences. 
I see already 3 requirements for labelling an isolation transformer and then 5 more for using it and 3 more about maintenance.

And the icing on the cake (copied &pasted from the BBC document): In most cases the RCD offers greater protection than a safety isolating transformer.

Ofcourse there are use cases for an isolation transformer but those are for using 230V equipment in moist/wet circumstances (*) and not for repair and R&D scenarios.
* In moist and wet circumstances you have to ask yourself if you aren't better off using battery or compressed air powered tools.

[Cerebus]

I don't use an isolation transformer when working on live equipment. I have a strict policy of doing the following though which is adhered to religiously:

1. Unplug, isolate.

A useful old acronym to remember here: SIDE
Switch off.
Isolate.
Dump (any charge holding circuits such as capacitors)
Earth (those same circuits so that any recharge from dielectric absorption gets dumped and any accidental reconnection of power trips breakers)

2. Attach test probes/equipment.
3. Plug in, power up.
4. Observe measurement without touching the unit.
5. Goto 2.

If your measurement equipment is properly earthed and PAT tested it should be fine.

I don't own differential probes nor do I want to spend any cash on them, so I used an inexpensive AD8130 with two normal 10x scope probes and a secondary switchable attenuator. Works fine, CMRR drops off pretty quick at about 50MHz and noise creeps in so you need to use LP filter in trigger and it'd probably explode if you put more than 500v diff through it but it does the job well enough. I'll detail the design at some point. Cost about £10 in total and runs off a 9v battery. The worst possible current leakage from mains is about 500uA with the design.

[Mark Hennessy]

I really don't see why people still try to defend the use of isolation transformers with more do's & don'ts than the 10 commandments which people also don't follow.

1. Only one DUT connected to the output.
2. Don't carry the earth through to the output socket.

Pretty sure that's fewer than 10

Then you should apply for a job at the BBC because you can reduce a 9 page document about safety to two sentences. 

I see already 3 requirements for labelling an isolation transformer and then 5 more for using it and 3 more about maintenance.

Labelling is an issue for the manufacturer, not the end user. What a peculiar point to make! It is absolutely the norm to label equipment - would you criticise a DMM for having the CAT ratings printed on the label?

I don't see 5 usage requirements. Over and above the 2 points I listed earlier, I suppose you can add the one about visual inspection - which you'd do with anything you use, surely?

Finally, the maintenance points apply to any piece of electrical equipment that you plan to plug in and use (visual inspection and periodic PAT test) - they are exactly the same thing you'd do with any bit of mains-powered gear, whether it's an isolation transformer or not.

And the icing on the cake (copied &pasted from the BBC document): In most cases the RCD offers greater protection than a safety isolating transformer.

Yes, and I have never disputed that - clearly you are not reading my posts properly.

For the 3rd time:

In an electronic workshop, isolation transformers are about allowing the safe connection of earthed test gear to a DUT - nothing more, nothing less. They are not about changing the probability or severity of an electric shock.

So absolutely, don't expect an isolation transformer to increase your safety. This is the big misconception that I frequently try to explain to people, and I find it amusing that you are arguing with me about this, because we both agree about it.

Where we differ is in your assertion that isolation transformers are completely worthless (and historical), and that an expensive differential probe is the only way. Whereas I assert that used correctly, an isolation transformer and a conventional 'scope probe is just as good - perhaps better in some scenarios. But people must know how to use them safely, and I'm happy to pass on that information.

Ofcourse there are use cases for an isolation transformer but those are for using 230V equipment in moist/wet circumstances (*) and not for repair and R&D scenarios.
* In moist and wet circumstances you have to ask yourself if you aren't better off using battery or compressed air powered tools.

Well, I have already given the example of 110V site transformers (with a grounded centre-tapped secondary). 230V versions also exist, as I said in my initial post. Those are intended to reduce the severity of an electric shock, but they are not intended to allow you to connect test gear to a live chassis - let's not confuse the two separate scenarios.

[Delta]

Well, don't take my word for it:
www.tek.com/dl/51W_10640_1.pdf

Oh, and I do take safety very seriously. When I was a teenager a good friend of mine electrocuted himself while tinkering with electricity.

That says not to use an isolation transformer to float the instrument, it absolutely does not say that an isolation transformer should not be used to supply the DUT!

VERY important distinction!

You didn't read far enough! In the table on the next page it says not to use an isolation transformer to make floating measurements -period-.

I did indeed read as far as that table - the whole table refers to instruments NOT DUTs!  (or should that be DsUT?)

The text from the preceding page should make it crystal clear that Tektronix are in no way saying that the use of isolation transformers to power DUTs is "An Unsafe And Dangerous Practice And Should Never Be Done!" :

Floating An Oscilloscope: A
Definition
“Floating” a ground referenced oscil-
loscope is the technique of defeating
the oscilloscope’s protective ground-
ing system – disconnecting “signal
common” from earth, either by defeat-
ing the grounding system or using an
isolation transformer.

Never attempt to defeat the protective grounding system of your oscilloscope by using an isolation transformer...

(emphasis mine)

Seriously mate, I think you've misunderstood Tek's gist there...!

[Shock]

Here is some RCD testing jump to 2:28:00 and wait for the results to come in.

[steverino]

Here is some RCD testing jump to 28:00 and wait for the results to come in.

Actually, the 240v shock is at 2:28:0, line on left hand, ground on right.  This fella might not be around to reach middle age.  Even after the shock, he keeps picking up the live board by the transformer in his left hand while holding a grounded panel in his right.

[Cerebus]

No visual check to ensure power has been fully removed.

Ooh! Product opportunity here for someone with access to a cheap injection moulding process.

Shooters use a device called a breach flag to ensure that weapons are handled safely. It's just a bright orange piece of plastic that has a rectangular 'flag' on the end of it and a more weapon specific part at the other end. It's inserted into the breach of a pistol or rifle in such a fashion that it's impossible to close the weapon's breach and therefore impossible to accidentally load or fire the weapon. Breach flag present - weapon is safe; breach flag absent - treat weapon as loaded and dangerous.

We need a 'breach flag' for IEC sockets. You remove the power cord and insert a moulded plastic plug that has a stalk with a flag on the end, the flag sticking up above the instrument/DUT shows it's safe and the plug part is a reminder to check the instrument for safety before replacing the power cord.

[RGB255_0_0]

https://youtube.com/watch?v=XBsQ3sZ45Fk

[Ian.M]

That video is *NOT* a good one.  See: https://www.eevblog.com/forum/projects/isolation-transformer-and-variac-safety/ which discusses it.

[SteveyG]

That video is terrifying - he seems to be oblivious to the fact he's handling a powered up PCB. 

I did a video a while ago on my isolation transformer - I mentioned floating DUTs or test equipment, but thinking about it now, it's quite a dangerous thing to suggest to the public - Isolation transformers and other protective devices REQUIRE you to understand exactly what you're doing and exactly what is at what potential. Almost an impossible thing to explain in video format.